1. Field of the Invention
The present invention relates generally to a frequency/voltage converting device and, more particularly, to a frequency/voltage converting device which is suitable for use in generating a DC voltage corresponding to the desired frequency from an input signal containing pulse trains of different frequencies.
2. Description of the Related Art
The standard television system adopted in Japan is the NTSC system in which a luminance signal (Y signal), similar to the signal used in an ordinary monochrome television system, is employed to represent the brightness of an image, while a subcarrier of approximately 3.58 MHz is used to transmit two chrominance signals for carrying colors. A composite video signal based on the NTSC system has a 21-H vertical blanking period (interval) which includes a first equalizing interval train of a 3-H period, a vertical synchronizing pulse interval of a 3-H period, a second equalizing pulse interval of a 3-H period, and a horizontal synchronizing pulse interval of a 12-H period. The term "H" used herein indicates the time period which elapses between the leading end of a particular scanning line and the leading end of the next scanning line (1/horizontal synchronizing frequency).
Referring to the operation of horizontal deflection in a television picture tube, an electron beam is oscillated horizontally by supplying a regular sawtooth current to deflection coils, thereby forming a regular raster. In general, the regular sawtooth current is generated in the following manner: A horizontal synchronizing pulse is converted into a DC voltage corresponding to its frequency (hereinafter referred to as a "F/V conversion") and the aforesaid sawtooth current is generated on the basis of the DC voltage. For this reason, it is necessary that the DC voltage be an accurate DC voltage containing no ripple voltage.
In such a television receiver, a F/V converting apparatus for use in effecting the F/V conversion includes a monostable multivibrator (hereinafter referred to as a "MM") whose input terminal is connected to an input terminal. The horizontal synchronizing pulses contained in a composite video signal are provided at the input terminal IN. Connected to the control part mounting terminal of the MM are a resistor and a capacitor, both of which cooperate to determine the width of an output pulse. A resistor, which constitutes a low-pass filter, is connected at one end to the output terminal of the MM. The other terminal of the resistor is connected to both an output terminal and one end of a capacitor. The other end of the capacitor is grounded.
The F/V converting device is arranged so that, if only the horizontal synchronizing pulse of a 1-H period is supplied to the input terminal, the MM outputs a pulse of a pulse width corresponding to the time constant determined by the resistor and the capacitor. The output pulse is smoothened by the low-pass filter and provided at the output terminal as a constant DC voltage containing no ripple voltage.
However, the DC voltage which is in practice provided at the output terminal has an output waveform including ripple voltages. This is because the composite video signal includes, adjacent to the horizontal synchronizing pulse interval containing pulses of a pulse width of 1 H, the equalizing pulse interval, the vertical synchronizing pulse interval and the further equalizing pulse interval, each of which interval contains pulses of a 1/2-H pulse width. More specifically, the magnitude of a DC voltage corresponding to the 1-H horizontal synchronizing pulse is +V, while an output voltage corresponding to the 1/2-H equalizing pulse is provided as a ripple voltage higher than +V. If a sawtooth current generated on the basis of the output voltage containing such ripple voltage is supplied to the deflection coils, regular horizontal deflection will be impossible. To achieve the regular horizontal deflection, for example, the time constant of the low-pass filter may be made large. However, the increase of the time constant may lead to the problem that the output voltage increases or that transient response characteristics deteriorates at the time of frequency switching in multi-scan operation or the like.